Reconfigurable baseband filter

ABSTRACT

A reconfigurable baseband filter for use in a multimode communication system is disclosed. One or more filter elements can each be configured as a plurality of sub-elements. The value of each of the filter elements can be varied by switching between at least two of the plurality of sub-elements. Switching noise within a desired passband can be reduced by switching at a rate that is greater than the desired passband. The switching noise in the passband can be further reduced by pseudo-randomly switching between the sub-elements. The filter can use a delta-sigma modulator to generate a pseudo-random switching signal.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/464,162, filed Apr. 21, 2003, entitled RECONFIGURABLEBASEBAND FILTER FOR MULTIMODE COMMUNICATION SYSTEMS, hereby incorporatedherein by reference in its entirety.

BACKGROUND OF THE DISCLOSURE

[0002] Filters are used in communications systems for many purposes. Inradio receivers, an active filter can be used to amplify signals atcertain frequencies and reject those at other frequencies as tosegregate wanted information from unwanted. In other words, filtersallow the radio receiver to provide the listener with substantially thedesired signal and substantially reject all other signals. In radiotransmitters, filters act to mitigate extraneous emissions out of theintended band. A filter may pass the signals in the intended band andattenuate those signals that lie outside of the intended band. The radiotransmitter can generate the signals in the desired band and will nottransmit other signals that might interfere with other spectrum users.

[0003] Analog filters used in the baseband section of RF receiverstypically satisfy stringent noise, linearity, power dissipation, andselectivity requirements. The existence of large interferers near thedesired signal frequency demands a high linearity and/or low noise inthe filters, impacting the distribution of gain and noise through thereceiver chain.

[0004] For these reasons, on-chip filters typically use a lot of diearea. A multi standard radio may support several standards, each withdifferent bandwidth requirements. Due to differing requirements for eachstandard, different filters are typically used. Multiple filters furtheraggravate the amount of die area used to implement filters. Multistandard radios that implement one filter for each standard add the costof having the multiple filters to the cost of the radio.

[0005] Wireless communication devices having the ability to operateunder different communication standards are presently in demand.Consumers of the multi-mode devices want increased levels ofperformance, but also desire low cost and small physical size. It isdesirable to improve the performance of a multi-mode device withoutincreasing the cost of the device and without increasing the size of thedevice.

BRIEF SUMMARY OF THE DISCLOSURE

[0006] A reconfigurable baseband filter for use in a multimodecommunication system is disclosed. One or more filter elements can eachbe configured as a plurality of sub-elements. The value of each of thefilter elements can be varied by switching between at least two of theplurality of sub-elements. Switching noise within a desired passband canbe reduced by switching at a rate that is greater than the desiredpassband. The switching noise in the passband can be further reduced bypseudo-randomly switching between the sub-elements. The filter can use adelta-sigma modulator to generate a pseudo-random switching signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The features, objects, and advantages of embodiments of thedisclosure will become more apparent from the detailed description setforth below when taken in conjunction with the drawings, in which likeelements bear like reference numerals.

[0008]FIG. 1A is a functional block diagram of a prior art fixedbandwidth filter

[0009]FIG. 1B is a plot of an embodiment of the fixed bandwidth filterof FIG. 1A.

[0010]FIGS. 2A-2B are a functional block diagrams of an embodiments of areconfigurable filter.

[0011]FIG. 2C is a plot of the frequency response of an embodiment of areconfigurable filter.

[0012]FIGS. 3A-3B are functional block diagrams of embodiments ofcommunication device integrated circuits having a reconfigurablebaseband filter.

[0013]FIG. 4 is a functional block diagram of an embodiment of abaseband circuit having a reconfigurable baseband filter.

[0014]FIG. 5 is a flowchart of an embodiment of a method of configuringa frequency response in a reconfigurable filter.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0015] A reconfigurable baseband filter and systems implementingreconfigurable baseband filters are disclosed. A filter can include oneor more elements configured to provide a filter response. Each of theelements can include a plurality of sub-elements. At least two of theplurality of sub-elements can be configured to be electrically inparallel. In one embodiment, an element includes two sub-elementsswitchably connected in parallel. In another embodiment, an element caninclude three or more sub-elements switchable connected in parallel.Increasing the number of sub-elements switchably connected in parallelmay reduce a switching noise contribution. For example, selectingbetween three parallel sub-elements may generate less noise thanswitching between two sub-elements.

[0016] The filter can switch between, for example, two parallelsub-elements to selectively connect one of the sub-elements to thefilter configuration. The filter selectively switches between parallelsub-elements to produce an element value that can be determined based inpart on the duration that each sub-element is connected within thefilter. The switches can be controlled using a pseudo random switchingcontrol signal to reduce switching noise contributions to the filteroutput.

[0017]FIG. 1A is a functional block diagram of a prior art single poleactive lowpass filter 100. The filter 100 includes an amplifier 110,such as an operational amplifier having an inverting input, anon-inverting input, and an output. The filter 100 also includes anumber of passive elements configured to provide a lowpass filterresponse.

[0018] A first resistor 120 couples a signal from a filter input to theinverting input of the amplifier 110. A second resistor 130 couples theoutput of the amplifier 1 10 to the inverting input. Similarly, acapacitor 140 connected in parallel to the second resistor 130 couplesthe output of the amplifier 110 to the inverting input. Thenon-inverting input of the amplifier 100 is coupled to a voltage common,which may be referred to as a return or ground.

[0019] The values of the first resistor 120, second resistor 130, andcapacitor 130 can be fixed to provide a lowpass filter response 150 asshown in FIG. 1B. The lowpass filter response 150 of FIG. 1B includes apassband and a stop band. The passband of the filter response 150 istypically characterized using the frequency at which the amplituderesponse falls below a predetermined value. Typical amplitude valuesused to characterize a filter response include −1 dB and −3 dB, althoughany predetermined value can be used to characterize a filter response.In the filter response 150 of FIG. 1B, the passband response ischaracterized by the −3 dB response point 152 that occurs at a frequencyof f_(3dB).

[0020] A particular filter 100 configuration can provide a singlepredetermined frequency response. Multi-mode communication devicestypically support a plurality of communication standards. The pluralityof communication standards can each have different signal bandwidths,signaling rates, or data rates. A filter having a frequency responseoptimized for a particular communication standard will likely not beoptimized for a different frequency standard.

[0021] For example, in a receiver configuration, the frequency responseof a baseband filter may substantially define a noise bandwidth of thereceiver. An excess noise bandwidth typically degrades the receivedsignal quality and may prevent distinguishing of signals that are near atheoretical sensitivity limit, regardless of the quality of the devicesused in the receiver design. Thus, a baseband filter in a receiverconfiguration can have a frequency response that is configured to passsubstantially the entire desired signal spectrum while rejectingsubstantially signals outside the desired signal spectrum.

[0022] A multi-mode receiver may, for example, be configured as amultiple mode wireless communication device. Several wirelesscommunication standards exist and each of the wireless standards mayutilize a different signal bandwidth or signaling rate.

[0023] For example, the signal bandwidth in a communication deviceoperating in accordance with the Advanced Mobile Phone System (AMPS)standard may use a baseband signal bandwidth of 30 kHz. A communicationdevice operating in accordance with a Code Division Multiple Access(CDMA) communication standard, such as Telecommunications IndustryAssociation/Electronics Industries Alliance (TLA/EIA) IS-95 may utilizea baseband signal bandwidth of 600 kHz or greater. Similarly, acommunication device operating in accordance with GSM standards mayoperate with a baseband signal bandwidth of 150 kHz. Similarly, devicesoperating in accordance with the Bluetooth communication standard mayoperate with a baseband signal bandwidth of 300 kHz. Similarly, a deviceoperating in accordance with Wideband CDMA (WCDMA) communicationstandard may operate with a baseband signal bandwidth of 3.8 MHz. In yetanother communication standard, a device operating in accordance with awireless Local Area Network (LAN) standard, such as the IEEE 802.11standard may operate with a baseband signal bandwidth of 9 MHz. Stillother communication standards may use other baseband signal bandwidths.

[0024] Thus, a multi-mode device operating in one or more of theabove-mentioned standards typically cannot use the same baseband filterto provide the baseband frequency response for more than onecommunication standard. A fixed frequency response filter, for examplethe filter 100 of FIG. 1A, that is optimized for AMPS communication willnot have sufficient bandwidth to support WCDMA. Similarly, a fixedfrequency response filter that is optimized for an IEEE 802.11 standardmay have a baseband bandwidth that is far greater than is required tosupport an AMPS communication mode. As mentioned above, such an excessbandwidth typically results in excess noise power and degradation of thereceiver sensitivity.

[0025] A typical method for a receiver to implement an optimizedbaseband signal bandwidth for multiple communication standards is to usemultiple different filters, with a filter for each desired frequencyresponse. Simply, multiple filters can be connected in parallel and thefilter having the desired frequency response is used while the othersare turned off.

[0026] The drawbacks of this approach include increased area and costneeded to implement multiple filters. Another drawback is that componentvalues, such as resistor and capacitor values, tend to vary from theirnominal value especially when implemented in integrated circuits. Tocombat the problem of component value variations, a large string ofresistors can be provided on-chip. A number of resistors in parallelwith each resistor varying slightly from the nominal value can beprovided on a chip to allow selection of a desired resistor value. Anexternal system can be used to directly or indirectly measure theresistors and determine which most closely approximates the desiredresistor value. However, the need for numerous components in parallelfurther aggravates the cost and area limitations.

[0027]FIG. 2A is a functional block diagram of a reconfigurable filter200. The reconfigurable filter 210 includes a filter module 210 havingmultiple filter elements configured to provide a desired filterresponse. The filter module 210 can also include one or more filterelements that are configurable based at least in part on a switchcontrol signal. The switch control signal configures the one or moreconfigurable elements, and the filter module 210 provides the filterresponse defined by the switch control signal.

[0028] In one embodiment, each configurable filter element includes apair of like filter elements of different value arranged in parallelwithin a filter configuration. Each of the like filter elements isselectively coupled to the filter by a switch configuration controlledby the switch control signal. In one embodiment, the configurable filterelement within a filter circuit can include two resistors arranged inparallel. Switches can be connected in series with the resistors toselectively connect a corresponding resistor to the filter circuit. Theswitch control signal can be used to selectively connect one or theother of the parallel resistors to the filter circuit. A filter elementresistor value can be generated by varying a percentage of time that oneor the other of the resistors is connected to the filter circuit.

[0029] The reconfigurable filter 200 also includes a switch controlmodule 220 configured to generate one or more switch control signalsthat can be used to configure one or more elements of the filter module210.

[0030] A switch interface logic module 240 can be used to couple theswitch control signals generated in the switch control module 220 to thecontrol inputs of the filter module 210. In some embodiments, the switchinterface logic module 240 may be integrated within the switch controlmodule 240 or the filter module 210.

[0031] The switch interface logic module 240 can be configured, forexample, to transform the control signal output from an output formatprovided by the switch control module 220 to a format desired by thefilter module 210. For example, the switch interface logic module 240may be configured to generate a differential signal from a single endedsignal. In another embodiment, the switch interface logic module 240 maybe configured to transform the control signal from a first logic levelto a second logic level. In still other embodiments, the switchinterface logic module 240 may process the switch control signal inother ways.

[0032] The switch control module 220 can contribute, by controlling theswitches in the filter module 210, switching noise contributed to thefilter module 210. Thus, it may be desirable to reduce the switchingnoise contribution.

[0033] In one embodiment, the switch control module 220 can beconfigured to generate a periodic switch control signal and may vary theduty cycle of the switch control signal. The frequency of the periodicswitch control signal can be configured to be outside a desired signalbandwidth in order to minimize switching noise contributions. Forexample, if the desired frequency response is a lowpass response havinga passband of 500 kHz, the frequency of the switch control signal may beselected to be greater than 500 kHz. The actual frequency of the switchcontrol signal may be selected based on a variety of factors, including,for example, a maximum operating speed of the switching circuits and anacceptable level of switching noise.

[0034] In another embodiment, the switch control module 220 can beconfigured to generate a pseudo random switch control signal. A pseudorandom switch control signal may generate a broader spectrum ofswitching noise and thus, the switching noise at any particularfrequency may be less than the peak switching noise generated by aperiodic signal. The switch control module 220 may, for example,implement a linear feedback shift register (LFSR) such as a maximumlength LFSR configured to provide a pseudo random output. The switchcontrol module 220 may then control the duty cycle of the pseudo randomoutput. The switch control module 220 may clock the pseudo randomcontrol signal at a rate that is outside the desired filter response tofurther minimize the switching noise contribution.

[0035] In other embodiments, the switch control module 220 can include amemory or register configured to store a pseudo random bit sequence. Theswitch control module 220 may then clock or shift the bits of the pseudorandom bit sequence to generate the pseudo random control signal. Thestored pseudo random bit sequence can have, for example, some knownproperty. For example, the stored pseudo random bit sequence can be aBarker code or a Walsh code that may be configured to provide a minimalnoise contribution to a desired signal.

[0036] In still another embodiment, the switch control module 220 can beconfigured to generate a pseudo random switch control signal, where thepseudo random output varies in order to achieve a desired filter elementvalue. The switch control module 220 can implement, for example, adelta-sigma modulator to generate a pseudo random switch control signal.The distribution of high and low logic levels output from thedelta-sigma modulator can be used to control the filter element value.

[0037]FIG. 2B is a detailed functional block diagram of an embodiment ofa reconfigurable filter 200. As will be described in further detail, thereconfigurable filter 200 embodiment of FIG. 2B is implemented in themanner described above for the reconfigurable filter of FIG. 2A.

[0038] The reconfigurable filter 200 includes a filter module 210 havingtwo control inputs that are clocked using a signal generated by theswitch control module 220. A switch interface logic module 240 couplesthe output of the switch control module 220 to the control inputs of thefilter module 210.

[0039] The filter module 210 in the embodiment of FIG. 2A is configuredas an analog active lowpass filter. However, the filter module 210 isnot limited to any particular configuration and may be configured as alowpass, highpass, bandpass, band reject, all pass, and the like, orsome other filter configuration. Additionally, the filter need not be anactive filter, but may be a passive filter. Although a baseband filteris described, the filter module 210 is not limited to a baseband filter,but can be, for example, an Intermediate frequency (IF) filter, a RadioFrequency (RF) filter, and the like, or some other frequency filter.Moreover, the filter need not be an analog filter, but may be a digitalfilter. Although the filter module 210 of FIG. 2B is a first orderfilter, the filter module is not limited to any particular filter order,and can be implemented as a filter having any desired filter order.

[0040] The filter module 210 includes an op amp 250 having inverting andnon-inverting inputs, and an output. A number of circuit elements areconfigured in conjunction with the op amp 250 to produce a lowpassfilter response. An input circuit element, or input resistor 260 couplesan input signal to the inverting input of the amplifier. A feedbackcircuit element or feedback resistor 270 couples the output of the opamp 250 to the inverting input. A feedback capacitor 280 connected inparallel to the feedback resistor 270 also couples the output of the opamp 250 to the inverting input. The non-inverting input of the op amp250 is coupled to a voltage common.

[0041] The input circuit element or input resistor 260 includes a numberof sub-elements, alternatively referred to as components. The inputresistor 260 is implemented as a plurality of like components switchablyarranged in parallel. Here, the number of like components is two, butany number of components may be used.

[0042] The sub-elements or components of the input resistor 260 includea first input resistor 262 in series with a first input switch 266coupling the filter input to the inverting input of the op amp 250. Theinput resistor 260 also includes a second input resistor 264 in serieswith a second input switch 268 coupling the filter input to theinverting input of the op amp 250. As can be seen, the first inputresistor 262 in combination with the first input switch 266 is connectedin parallel to the second input resistor 264 in combination with thesecond input switch 268. Thus, one or both of the input resistors can beconnected to the filter circuit if the corresponding input switches 266and 268 are switched in or conducting. The input switches 266 and 268can be controlled with signals of opposite polarity such that one of thefilter components is coupled to the filter circuit at any instant oftime. The input switches 266 and 268 can thus effectively operate as asingle pole double throw switch that selectively switches between one orthe other of the filter components.

[0043] If the first and second input resistors 262 and 264 are chosen tobe different values, the effective value of the input resistor 260 canbe determined according to the percentage of time that each of the firstand second input resistors 262 and 264 are switched into the filtercircuit. Thus, the value of the input resistor 260 can be determined bythe formula:

R=P1×R1_(A)+(1−P1)×R1_(B)

[0044] In the formula P1 represents the fraction of time that the firstinput resistor (R1_(A)) 262 is switched into the filter circuit and(1−P1) represents the fraction of time that the second input resistor(R1_(B)) 264 is switched into the filter circuit. The formula assumesthat the time the first input resistor 262 is switched into the circuitis substantially exclusive of the time the second input resistor 264 isswitched into the filter circuit.

[0045] Thus, the two component values define the extremes over which theinput resistor 260 may vary, given the switching scheme described above.For example, a fixed switch state with the first input resistor 262switched into the filter circuit results in the input resistor 260having the value of the first input resistor 262. Similarly, a fixedswitch state with the second input resistor 264 switched into the filtercircuit results in the input resistor 260 having the value of the secondinput resistor 264. The value of the input resistor 260 can thus bevaried by varying a switching control signal to vary the fraction oftime that each of the components is connected to the filter circuit.

[0046] The feedback circuit element 270 is configured similar to theinput circuit element 260. The feedback circuit element 260 includes afirst feedback resistor 272 in series with a first feedback switch 276coupling the filter output to the inverting input of the op amp 250. Thefeedback resistor 270 also includes a second feedback resistor 274 inseries with a second feedback switch 278 coupling the filter output tothe inverting input of the op amp 250. Varying the fraction of time thateach of the feedback resistors 272 and 274 is connected to the filtercircuit varies the value of the feedback resistor 270.

[0047] Effectively, the first set of filter components configure afilter having a first filter response and the second set of filtercomponents configure a filter having a second filter response.Typically, the first and second filter responses are different. Theswitch control signal can be used to selectively switch between thefirst and second sets of filter components to produce a filter having afilter response between the first and second filter responses.

[0048] Thus, the values of the input resistor 260 and the feedbackresistor 270 may be varied by varying the switch control signal. Thevalues may be varied to allow the single reconfigurable filer 200provide an optimized frequency response for multiple operating modes andmultiple communication standards. For example, the reconfigurable filtercan be configured to provide at least a lowpass filter response having apassband of substantially 150 kHz, 500 kHz, 3.8 MHz, and 9 MHz.

[0049] The input resistor 260 and the feedback resistor 270 are shown ascontrolled by the same switch control signal. This configuration canresult in the two elements 260 and 270 varying in the same proportion.However, such proportionate variation is not a requirement and may notbe desired in other embodiments. For example, in a multiple pole filter,the values of individual filter elements may be varied according toindependent switch control signals. In still other embodiments, one ormore filter elements may share the same switch control signal whileother filter elements may use one or more independent switch controlsignals.

[0050] In one embodiment, a plurality of independent switch controlsignals can be used to vary filter element values in a multi-pole filterto change a filter frequency response. In another embodiment, one ormore switch control signals can be used to change a filtercharacteristic. For example, in addition to changing the frequencyresponse of a filter, the switch control signals can vary the elementvalues to change a Butterworth filter response to a Chebyshev filterresponse.

[0051] Although the filter elements 260 and 270 are shown as resistiveelements, the filter elements are not limited to resistive elements. Forexample, a configurable filter element can be a capacitive element orcan be an inductive element. Furthermore, the configurable filterelement is not limited to a passive element, but can include activefilter elements.

[0052] In one embodiment, the filter element can be configured to be apassive capacitive filter element having two capacitors switchablyconnected in parallel. In another embodiment, the filter element can beconfigured to be a passive inductive filter element having two inductorsswitchably connected in parallel. In still another embodiment, thefilter element can be configured to be an active filter element havingtwo transconductor components (g_(m)), alternatively referred to astransconductance elements, switchably connected in parallel. In stillother embodiments, other filter components can be switchably connectedin parallel.

[0053] Although the filter elements are shown as having two componentsconnected in parallel, each filter element can have more than twocomponents connected in parallel. Further, more complex components,having more than one component and more than one component type, may beused.

[0054] The switch control module 220 can be configured as a delta-sigmamodulator in order to generate a pseudo random switch control signal.The switch control module 220 includes a difference amplifier 222 orsummer having inverting and non-inverting inputs. The output of thedifference amplifier 222 is coupled to the input of an integrator. Theoutput of the integrator 224 is coupled to a latch 226 having a clockinput used to clock an input value to the output of the switch controlmodule 220. The clock signal may be generated by an oscillator or systemclock (not shown) that has a frequency greater than the highest desiredpassband frequency of the reconfigurable filter 200. The output of thelatch 226 is also coupled to an input of a one bit Digital to AnalogConverter (DAC), which may be an inverter 228. The output of theinverter 228 is coupled to the inverting input of the differenceamplifier 222.

[0055] The delta-sigma modulator is shown as a first order delta-sigmamodulator. However, other embodiments may use delta-sigma modulators ofother orders such as, for example, a second, third, fourth, or fifthorder delta-sigma modulator.

[0056] The pseudo random output of the delta-sigma modulator can bedetermined in part based on the value at the non-inverting input of thedifference amplifier 222. A variable voltage source 230 may be used asan input to the difference amplifier 222. Thus, the value of thevariable voltage source 230 can be used to determine the pseudo randomoutput of the delta sigma converter, and thereby the frequency responseof the reconfigurable filter 200.

[0057] The variable voltage source can be, for example, a continuouslyvariable voltage source having a control input, a discretely variablevoltage source, a plurality of fixed voltage sources switchably coupledto an output, a plurality of control words coupled to a DAC, and thelike, or some other manner of providing a variable voltage output.

[0058] The output of the switch control module 220 is coupled to theinput of the switch interface logic module 240. Because the filtermodule 210 in this embodiment uses mutually exclusive switch pairs, theswitch interface logic module 240 may be configured as logic thattransforms a single switch control signal to two switch control signalsof opposite polarity. The switch interface logic module 240 may thus beconfigured as an inverter 242 coupled in parallel to a non-invertingthrough path to create two switch control output signals that are ofopposite polarity.

[0059]FIG. 2C is a plot of the frequency response of a reconfigurablebaseband filter, such as the reconfigurable filter of FIGS. 2A and 2B. Afirst frequency response 290 can represent the frequency response of thereconfigurable filter when a first set of filter elements is connectedto the filter circuit. For example, in the reconfigurable filter of FIG.2B, the frequency response may correspond to the condition where thefirst input resistor 262 and first feedback resistor 272 are connectedto the filter circuit. That is, the first frequency response 290 maycorrespond to the switch control signal selecting a first filterconfiguration for substantially 100% of the time.

[0060] A second filter frequency response 294 may occur when a secondset of filter elements is connected to the filter circuit. For example,in the reconfigurable filter of FIG. 2B, the second frequency response294 may correspond to the condition where the second input resistor 264and second feedback resistor 274 are connected to the filter circuit.The second frequency response 294 may thus correspond to the switchcontrol signal selecting a second filter configuration for substantially100% of the time.

[0061] Thus, by configuring the switch control signal to selectivelyswitch between the first and second filter configurations, the filtercan provide almost any frequency response between the first frequencyresponse 290 and the second frequency response 294. For example, a thirdfrequency response 292 mid-way between the first and second frequencyresponses, 290 and 294, can be produced by equally switching between thetwo sets of filter elements. The first set of filter elements call beswitched in to the filter circuit for approximately 50% of the time andthe second set of filter elements can be switched into the filtercircuit for the remainder of the time. Thus, varying the fraction oftime that each set of filter elements is switched into the filtercircuit varies the frequency response.

[0062]FIG. 3A is a functional block diagram of embodiments of a receiverfront end module 300, such as a multi-mode receiver front end that maybe implemented in a receiver integrated circuit. The receiver front endmodule 300 can include an input that can be configured to receivesignals from, for example, an antenna (not shown). The receiver frontend module 300 may include an amplifier, such as a Low Noise Amplifier(LNA) configured to amplify the received signal. The amplifier may havesufficient frequency response to allow operation over multiplecommunication bands supporting communication standards. The output ofthe LNA 210 can be coupled to the input of a mixer 320 or some othertype of frequency conversion stage. A Local Oscillator (LO) 324, whichis typically implemented as a tunable LO, can be coupled to a LO port ofthe mixer 320. The frequency of the LO 324 can be tuned to frequencyconvert the received signal to an Intermediate Frequency (IF) orbaseband. The output of the mixer 320 can be coupled to a bufferamplifier 330 that is typically used to amplify the frequency convertedoutput and may also be used to provide a constant impedance load to themixer 320. The output of the buffer amplifier 330 can be coupled to theinput of a reconfigurable filter 200, such as the reconfigurable filter200 of FIGS. 2A and 2B. The output of the reconfigurable filter 200 canbe the output of the receiver IC 300 and can be configured to be coupledto a baseband processor (not shown).

[0063] Thus, a single receiver front end IC may be used for a receiversupporting multiple communication standards. The reconfigurable filter200 allows the module 300 to have optimized filter response withoutrequiring substantially greater die area to be allocated to the filter.Additionally, the reconfigurable filter using small die area can resultin a lower module cost.

[0064]FIG. 3B shows a functional block diagram of a similar multi-modetransmitter module 302, which may be, for example, a transmitter IC. Themulti-mode transmitter module 302 can include a reconfigurable filter200, such as the reconfigurable filter shown in FIGS. 2A and 2B. Thereconfigurable filter 200 can be configured to accept an input signalfrom, for example, a baseband module (not shown). The signal can be, forexample, a baseband signal or an IF signal. The output of thereconfigurable filter 200 can be coupled to an input of a driveramplifier 340 that amplifies the filtered signal. The output of thedriver amplifier can be coupled to the input of a mixer 330. A tunableLO can be used to drive the LO port of the mixer 350 to, for example, upconvert the signal to an RF frequency. The output of the mixer 350 canbe coupled to a power amplifier 360 that is configured to transmit thefrequency converted signal via an antenna (not shown).

[0065] The multi-mode receiver 300 can be implemented on the same IC asthe multi-mode transmitter 302 to provide a multi-mode transceiver IC.The multi-mode transceiver IC can be used to support multiplecommunication standards, such as wireless telephone standards orwireless LAN standards.

[0066]FIG. 4 is a functional block diagram of a baseband module 400having a reconfigurable filter 200, such as the reconfigurable filter ofFIGS. 2A and 2B. The baseband module 400 can be, for example, a basebandIC for use in a multi-mode communication device. The baseband module 400can include a reconfigurable filter 200 configured to receive an inputsignal such as, for example, a signal from a receiver front end. Theoutput of the reconfigurable filter 200 can be coupled to a demodulator410 that is configured to demodulate the received signal according toone of a plurality of supported communication standards. The output ofthe demodulator 410 can be coupled to the input of a baseband processor420 configured to further process the demodulated signal. The basebandprocessor 420 can, for example, generate a voice signal or displaysignal from the received information. The baseband processor 400 mayalso be configured to control the frequency response of thereconfigurable filter 200 by generating a mode select signal or someother bandwidth control signal. For example, the baseband processor maycontrol the variable voltage source of a switch control module in thereconfigurable filter 200 to configure a particular frequency responseassociated with a particular communication standard.

[0067]FIG. 5 is a flowchart of a method 500 of configuring the frequencyresponse of a reconfigurable filter. The reconfigurable filter can be,for example, the reconfigurable filter of FIGS. 2A and 2B. The method500 can be performed, for example, within the RF or baseband ICs shownin FIGS. 3 and 4.

[0068] The method 500 begins at block 510 where the IC determines thebroadest frequency response provided by the reconfigurable filter. Inone embodiment, each IC or each lot of ICs can be characterized formaximum and minimum frequency responses and the value of the frequencyresponses stored in memory or storage device within the IC. An ICprocessor can then determine a maximum, or broadest, frequency responseby reading the memory location holding the previously determined value.

[0069] The IC then proceeds to block 520 and determines the narrowest,or minimum, frequency response provided by the reconfigurable filter.The IC can determine the minimum bandwidth, for example, using the samemethod used to determine the broadest frequency response.

[0070] The IC then proceeds to block 530 and determines the desiredfrequency response. Typically, the desired frequency response is limitedto occurring between the broadest and narrowest frequency responses. Inone embodiment, the IC can be configured to operate in accordance with apredetermined number of communication standards. Each communicationstandard may have an associated bandwidth that can be provided by thereconfigurable filter. The IC may generate or receive a control signalthat indicates the mode of operation, and thus a corresponding desiredfrequency response.

[0071] After determining the desired frequency response, the IC proceedsto block 540 and determines a fractional switching time. The fractionalswitching time can represent the fraction of time that a first set offilter elements is switched into the filter circuit. The remainder oftime can represent the time that a second set of filter elements isswitched into the filter circuit. The fractional switching time can berepresented by a digital value, voltage, or current. In thereconfigurable filter of FIG. 2B, the fractional switching time can berepresented by the voltage output of the variable voltage source coupledto the delta-sigma modulator.

[0072] The IC then proceeds to block 550 and switches the filterelements at a rate determined by the fractional switching time. The ICcan switch the filter elements using a periodic or a pseudo randomswitching pattern. In the reconfigurable filter embodiment of FIG. 2B, adelta-sigma modulator provides a pseudo random switch control signalthat switches the filter elements at the fractional allocation used toprovide the desired frequency response.

[0073] The IC then proceeds to decision block 560 to determine if a newfrequency response is desired. A change in the frequency response may bedesired, for example, if the IC changes to a different communicationmode.

[0074] If a new frequency response is desired, the IC returns to block530 to determine the new desired frequency response. However, if nochange in the frequency response is desired, the IC returns to block 550and continues to switch the filter elements at the determined fractionalallocation.

[0075] Thus, by determining a fractional allocation between twofrequency response extremes, the IC can provide a filter having almostany desired frequency response between a maximum and minimum frequencyresponse. The ability to configure multiple frequency responses usingfilter elements that comprise a pair of like sub-elements allows afilter to be produced in a relatively small die area and at a lower costrelative to having multiple filters connected in parallel.

[0076] Thus, a reconfigurable filter and a method of reconfiguring afrequency response of a filter have been disclosed. The reconfigurablefilter can include one or more elements arranged in a filterconfiguration. One or more of the filter elements can be a configurablefilter element. Each configurable element can include a plurality oflike filter elements switchably connectable to the filter circuit. Inone embodiment, each configurable filter element includes a pair of likefilter components switchably connected in parallel. The value of theconfigurable element can be modified by switching between the pair offilter components.

[0077] A switch control module generating a switch control signal cangenerate a periodic switch control signal or can generate a pseudorandom switch control signal. The switch control signal can beconfigured to switch between the filter components at a rate that ishigher than the desired filter passband.

[0078] The switch control module can generate a pseudo random switchcontrol signal using, for example, a delta-sigma modulator.Alternatively, the switch control module can generate a periodic switchcontrol signal having a duty cycle that varies in order to vary thefrequency response of the filter. In still another embodiment, theswitch control module can generate a pseudo random switch control signalthat has a duty cycle that varies in order to vary the frequencyresponse of the filter.

[0079] The above description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the disclosure.Various modifications to these embodiments will be readily apparent tothose skilled in the art, and the generic principles defined herein maybe applied to other embodiments without departing from the scope of thedisclosure. Thus, the disclosure is not intended to be limited to theembodiments shown herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

What is claimed is:
 1. A reconfigurable filter comprising: a pluralityof elements including a configurable element and configured to provide afilter circuit, the configurable element including at least two filtercomponents and a switch configured to selectively couple one of the atleast two filter components to the filter circuit; and a switch controlmodule configured to generate a switch control signal to control theswitch in the configurable element to selectively switch between twofilter components, a value of the configurable element based in part onthe switch control signal.
 2. The filter of claim 1, wherein the filtercircuit comprises an active filter circuit.
 3. The filter of claim 1,wherein the filter circuit comprises a passive filter circuit.
 4. Thefilter of claim 1, wherein the plurality of elements is configured toprovide a baseband filter.
 5. The filter of claim 1, wherein theconfigurable element comprises two like components of different valuesand the switch, and wherein the switch is configured to couple one ofthe two like components to the filter circuit.
 6. The filter of claim 1,wherein the configurable element comprises two like components ofdifferent values, each of the like components selected from the listcomprising a resistor, a capacitor, an inductor, and a transconductanceelement.
 7. The filter of claim 1, wherein the switch control modulegenerates the switch control signal having a switch control frequencygreater than a passband frequency of the filter.
 8. The filter of claim1, wherein the switch control module generates the switch control signalhaving a switch control frequency that lies outside a passband of thefilter.
 9. The filter of claim 1, wherein the switch control modulegenerates a periodic switch control signal.
 10. The filter of claim 9,wherein the switch control module varies a duty cycle of the periodicswitch control signal.
 11. The filter of claim 1, wherein the switchcontrol module generates a pseudo random switch control signal.
 12. Thefilter of claim 1, wherein the filter circuit comprises a lowpassfilter, and the switch control module is configured to generate theswitch control signal to produce one of a plurality of predeterminedfrequency responses.
 13. A reconfigurable filter comprising: a firstconfiguration of elements configured to provide a first filter response;a second configuration of elements configured to provide a second filterresponse different from the first filter response; at least one switchconfigured to selectively switch between the first configuration and thesecond configuration; and a switch control module configured to generateat least one switch control signal comprising a pseudo random sequenceto control the position of the at least one switch.
 14. Thereconfigurable filter of claim 13, wherein the switch control modulecomprises a pseudo random modulator.
 15. The reconfigurable filter ofclaim 13, wherein the switch control module comprises a delta-sigmamodulator.
 16. The reconfigurable filter of claim 15, wherein the deltasigma modulator comprises a latch clocked at a rate greater than apassband frequency of the first filter response.
 17. The reconfigurablefilter of claim 15, wherein the delta sigma modulator comprises a latchclocked at a rate that lies outside a passband of the first and secondfilter responses.
 18. A reconfigurable filter comprising: a configurableelement comprising: a first filter component in series with a firstswitch; and a second filter component in series with a second switch,the second filter component and second switch connected in parallel withthe first filter component and first switch; at least one fixed filterelement arranged with the configurable element to produce a filtercircuit; and a switch control module configured to generate a pseudorandom switch control signal to control the first and second switches toselectively switch between the first and second switch components. 19.The reconfigurable filter of claim 18, wherein a value of theconfigurable element is based at least in part on a fractionalallocation of the pseudo random switch control signal to a first signallevel.
 20. A reconfigurable filter comprising: at least one configurableelement having a value based in part on a fractional period in which acontrol signal is at a first signal level; and a filter element coupledto the at least one configurable element to produce a filter circuit.21. A method of configuring a filter response, the method comprising:determining a first filter response corresponding to a first switchconfiguration of at least one configurable element; determining a secondfilter response corresponding to a second switch configuration of the atleast one configurable element; determining a desired filter responsehaving a frequency response between the first filter response and thesecond filter response; selectively switching between the first switchconfiguration and the second switch configuration to produce the desiredfilter response.
 22. The method of claim 21, wherein the first filterresponse comprises a broad filter configuration.
 23. The method of claim21, wherein the second filter response comprises a narrow filterconfiguration.
 24. The method of claim 21, further comprising:determining a fractional switching time that produces the desired filterresponse; and selectively switching between the first switchconfiguration and the second switch configuration using a pseudo randomswitching signal that controls the switches to the first switchconfiguration for the fractional switching time.
 25. An RF integratedcircuit having a multimode frequency response, the circuit comprising:an amplifier configured to receive an RF signal; a mixer coupled to theoutput of the amplifier and configured to frequency convert the RFsignal; and a reconfigurable filter coupled to an output of the mixer,the reconfigurable filter comprising: a plurality of elements includinga configurable element and configured to provide a filter circuit, theconfigurable element including at least two filter components and aswitch configured to selectively couple one of the at least two filtercomponents to the filter circuit; and a switch control module configuredto generate a switch control signal to control the switch in theconfigurable element to selectively switch between two filtercomponents, a value of the configurable element based in part on theswitch control signal.
 26. A baseband processor integrated circuithaving a multimode frequency response, the integrated circuitcomprising: a reconfigurable filter comprising: at least oneconfigurable element having a value based in part on a fractional periodin which a control signal is at a first signal level; and a filterelement coupled to the at least one configurable element to produce afilter circuit; a demodulator coupled to the output of thereconfigurable filter; and a baseband processor coupled to the output ofthe demodulator and configured to generate a mode select signal thatcontrols, in part, the fractional period in which the control signal isat the first signal level.